Abstract
Analyses by sodium dodecyl sulphate-polyacrylamide gradient-gel electrophoresis and Western blotting of the proteins from boiled cell samples from all growth phases of yeast cultures, and from all stages of extract preparation indicate that the smaller subunit (s-monomer), which is found in purified glycogen phosphorylase (EC 2.4.1.1) from baker's yeast, is not present in the living cell. It is observed in extracts of Saccharomyces carlsbergensis and S. cerevisiae after incubation at ambient temperatures or even after storage in the frozen state at -25 degrees C. Its formation is sensitive towards pepstatin A, and it is absent from extracts of several mutants of S. cerevisiae that do not contain active proteinase yscA (EC 3.4.33.6). When purified proteinase yscA is added to the extracts of these mutants, the formation of s-monomer is restored. When the proteinase yscA-deficient strains are grown with a reduced amount of complex nitrogen compounds, the slightly smaller sc-monomer is formed in their extracts. This event must be attributed to a different proteinase, since it is sensitive towards p-hydroxymercuriphenylsulphonate, but not towards pepstatin A. The N-terminal amino acid of the sc-monomer was found to be blocked, as in the case of the native l-monomer, but not the s-monomer.
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